With a free account on ROBLOX you are given access to a Game Place, which gives you access to limited features. With full Personal Server access you can get additional features and benefits.

Roblox is an online platform that features private servers that players can purchase for supported games to play with their friends or selected people. The private server makes the farming process ...

The last way to join a specific Roblox game is if players have friends playing in that server. They are able to join any friend's game if they have the "Join In-game" setting enabled.